1. Field of the Invention
This invention relates to a positive temperature coefficient thermistor device, more particularly to an encased positive temperature coefficient thermistor device for use in the demagnetization circuit of a color television set, a color video display or the like.
2. Prior Art Statement
A typical positive temperature coefficient thermistor device for use in the demagnetization circuit of a color television set or the like is taught by Japanese Utility Model Publication Sho 59-15171.
As shown in FIG. 5, this conventional positive temperature coefficient thermistor device, denoted by reference numeral 1, has a positive temperature coefficient thermistor element 3 for demagnetization and a positive temperature coefficient thermistor element 4 for heating, which are enclosed in an insulating case 2 and opposed to each other through a heat conducting terminal member 6 of a common terminal plate 5. The positive temperature coefficient thermistor element 3 and the positive temperature coefficient thermistor element 4 are clamped between the spring terminal members 8 of a pair of terminal plates 7. The common terminal member 9 of the common terminal plate 5 and the terminal members 10 of the terminal plates 7 extend to the exterior of the insulating case 2. The common terminal plate 5 is formed of a metal material exhibiting a thermal conductivity of 0.02-0.09 cal/cm sec .degree.C. and a thickness of 0.05-0.3 mm.
The positive temperature coefficient thermistor device 1 is used as shown in FIG. 6, for example. Specifically, the common terminal member 9 of the common terminal plate 5 and the terminal member 10 of the terminal plate 7 associated with the positive temperature coefficient thermistor element 4 for heating are connected with opposite ends of a series circuit constituted of a demagnetization power source 11 (providing an ac 120 V output, for example) and a switch 12, and a coil 13 is connected across the terminal members 10 of the terminal plates 7,7 associated with the positive temperature coefficient thermistor elements 3 and 4. When the switch 12 is closed, the voltage of the demagnetization power source 11 is applied across the positive temperature coefficient thermistor element 4 for heating, causing it to produce heat. This heat warms the positive temperature coefficient thermistor element 3 for demagnetization, causing its resistance to increase. The demagnetization current flowing through the demagnetization coil 13 thus decreases, whereby it becomes possible to cancel the magnetism around a color TV picture tube, for example.
The common terminal member 9 of the common terminal plate 5 in the positive temperature coefficient thermistor device 1 is exposed to high temperatures. The temperature reached, while varying with the thickness of the common terminal plate 5, is as high as 87.degree.-101.degree. C. when the common terminal plate 5 is formed of stainless steel having a thermal conductivity of 0.03 cal/cm sec .degree.C. and as high as 93.degree.-108.degree. C. when it is formed of nickel silver. Because of this, the mounting panel on which the positive temperature coefficient thermistor device 1 is mounted and electronic components located in vicinity of the positive temperature coefficient thermistor device 1 tend to suffer heat degradation over a short period of time.
Typical of the arrangements of the positive temperature coefficient thermistor device 1 that have been developed for preventing the common terminal member 9 of the common terminal plate 5 from being heated to a high temperature is that disclosed by Japanese Patent Public Disclosure Hei 1-220403 and shown in FIG. 7.
The disclosed arrangement involves the provision of a row of through-holes 14 near the base of the common terminal member 9 so as to form a plurality of heat conduction bottlenecks 15. The heat conduction bottlenecks 15 suppress passage of the heat absorbed by the heat conducting terminal member 6 and thus prevent the common terminal member 9 from rising to a high temperature.
However, since the heat conduction suppressing effect of the heat conduction bottlenecks 15 is low, the maximum temperature of the common terminal member 9 of the positive temperature coefficient thermistor device 1 arranged in this manner is reduced only to around 74.degree. C. so that the heat dissipated from the common terminal member 9 still causes rapid degradation of solder joints, the mounting panel and near-by electronic components. Moreover, power consumption is increased.
Another problem is that the provision of the through-holes 14 beneath the heat conducting terminal member 6 of the common terminal plate 5 means that the heat conducting terminal member 6 includes a region that is at least as high as the diameter of the through-holes 14. This means that an empty space is formed between the bottom of the insulating case 2 and the positive temperature coefficient thermistor element 3 for demagnetization or the positive temperature coefficient thermistor element 4 for heating and that, therefore, the size of the positive temperature coefficient thermistor device 1 is increased by the amount of this space. This is undesirable in a field where high priority is placed on size reduction.
The reference numerals not mentioned in the foregoing explanation with reference to FIG. 7 indicate the same components as the corresponding numerals in FIG. 5.
There is thus a need for a positive temperature coefficient thermistor device of a structure which ensures that as little as possible of the heat absorbed by the heat conducting terminal member 6 will be conducted to the common terminal member 9 but which does not increase the size of the device.
This invention is intended to overcome the aforesaid problems and provides a positive temperature coefficient thermistor device which effectively suppresses the conduction of heat from the heat conducting terminal member to the common terminal member without increasing the size of the device.